It always reminds me of a fire brigade or an army. You want the firefighters to have lots of slack so that they will be available when there is a fire. On the other hand, having people sitting around "being ready" is costly and there is some optimum amount you need.
The timescale makes a huge difference here. Firefighters need to be next to their equipment to respond to an emergency. US army reservists “One Weekend a Month, Two Weeks a Year” is understating things, but they can hold down a full time job when not called up.
What I meant was that both professions are good examples of "slack-full" professions. They can still be considered to be doing their job well even if they're not actively fighting (either fires or the enemy). If they're consistently inactive they may get a reduction in numbers, but I've never seen anyone seriously suggest to abolish the fire brigades.
